Gerd Muller, who turns 70 next month, is being treated for Alzheimer's disease, a type of dementia causing problems with memory, thinking and behaviour.
Muller, nicknamed 'Der Bomber' scored 68 goals in 62 games for what was then West Germany - a record that was only beaten in 2014 by Miroslav Klose during the World Cup. He won the Golden Boot for netting 10 goals in the 1970 World Cup, and scored the winning goal in the 1974 final when West Germany defeated Netherlands 2-1.
According to Bayern chairman Karl-Heinz Rummenigge, Muller is "one of the all-time greats of world football."
Muller netted 14 goals during World Cup appearances, putting him third on the all-time list, behind Klose (16) and Brazil's Ronaldo (15). In 1972, he scored 85 times, and held the record for most goals in a calendar year until Lionel Messi beat his total in 2012.
Rummenigge praised Muller's humility and compassion, and noted his contribution to the Bayern Munich club: "He was a fantastic teammate and is a friend. Gerd will always enjoy a place in the Bayern family.
"After he ended his playing career, he brought his experience as a coach of youngsters to the club, helping define the likes of world champions Philipp Lahm, Bastian Schweinsteiger and Thomas Muller, and we are also grateful to him for this."